Conventional sample vials for automatic samplers typically comprise a small glass vessel which may be closed by a rubber septum held in place by a cap attached to the vessel by a screw thread or by crimping. After sealing, a number of such vials may be loaded into an autosampler which extracts a sample from each vial in turn and sends it for analysis by any desired analytical instrument, for example a chromatograph or spectrometer. Both gaseous and liquid samples may be analysed in this way. In some cases, the vial may be partially filled with a liquid sample but only gas present in the space above the liquid (the headspace gas) may be sampled. Typically, the autosampler will comprise a syringe whose needle, under computer control, is caused to pierce the septum of a selected vial so that a sample of gas or liquid can be withdrawn through the needle and analysed. The septum maintains a gas-tight seal around the needle during this process to prevent contamination. Means are provided to align a second vial with the syringe after the first one has been sampled so that many vials may be sampled automatically in sequence. Means may also be provided for cleaning the syringe between samples.
For the majority of samples, silicone rubber septa may be used to close the vial, but where there is a possibility of reaction of the vial contents with a conventional septum, a septum lined with an inert material, such as PTFE or Teflon, may be employed. These are used with the PTFE lining in contact with the vial contents so that the contents are protected from contact with the rubber septum while the rubber portion still provides a good seal around the needle. A septum made from a solid material such as PTFE is generally incapable of providing an adequate seal around the needle. Unfortunately, lined septa of this type have a limited lifetime because the lining is usually damaged by insertion of the needle, leaving an area of silicone rubber in contact with the vial contents.
This problem has been addressed by the provision of sample vial closure devices which incorporate a shut-off valve as well as a septum. When the valve is opened, a syringe needle may be passed through it to allow the vial contents to be sampled. A silicone rubber septum or seal, through which the needle must also pass, is also provided to seal the vial while the needle is inserted because the valve itself is unable to seal to the needle. Vial closure devices of this type are disclosed in U.S. Pat. Nos. 3,757,981 and 3,603,471, and are commercially available (for example, “Mininert” valves available from Sigma-Aldrich Company Ltd.).
The vial closure device taught in U.S. Pat. No. 3,603,471 comprises a first tube fitted to the screw cap of a conventional glass vial, through which tube a syringe needle may be inserted. A valve stem comprising a second tube is disposed in a bore perpendicular to the axis of the first tube and may be rotated therein by means of an external handle. The second tube comprises a hole perpendicular to its axis which may be aligned with the bore of the first tube by rotation by the handle to open the valve and allow unobstructed passage of a syringe needle through the first tube into the vial. A plug type rubber septum is inserted into the bore of the second tube so that it is pierced by the syringe needle as it is inserted through the device when the valve is open. In this way the contents of the vial are exposed to the septum only for the short time that the valve is open, minimising contamination. U.S. Pat. No. 3,757,981 teaches a variation on this design in which the second tube is replaced by a rod having a bore perpendicular to its axis which may be aligned with the bore of the first tube by sliding the rod. In this device, the septum is placed in the entrance of the first tube. The currently available “Mininert” valves are of this general design.
In some analyses it is necessary to sample only the headspace gas from a vial. One example of this is the measurement of the isotopic composition of hydrogen and/or oxygen in samples of water. In this method a sample of water is placed in a vial which is then sealed. A quantity of carbon dioxide (for oxygen analysis) or hydrogen (for hydrogen analysis) of accurately known isotopic composition is then injected into the vial and allowed to equilibrate with the water. After a suitable time has elapsed, a sample of the headspace gas is withdrawn and its isotopic composition is determined by mass spectrometric analysis. The original isotopic composition of the hydrogen or oxygen present in the water can then be determined from the change in the isotopic composition of the headspace gas during equilibration. Unfortunately, prior attempts to automate this process have not been successful because of losses or contamination of the headspace gas arising from contact with the rubber septum (or a lined septum after it has been punctured to admit the equilibration gas) during the equilibration process. Prior types of valved vials have also proved unsatisfactory, firstly because they cannot easily be used with conventional autosamplers, requiring manual operation of the valve handle, and secondly because the surface area of the material from which the devices are made (PTFE) is so large that contamination or loss of the headspace gas can still occur.